To identify a specific hydraulic cylinder and detect an extent of damages to the seal mechanism thereof very easily and properly, upon the occurrence of damages to the seal mechanism of a certain hydraulic cylinder among a plurality of the hydraulic cylinders installed on a machine.

The O-rings 20a and 23a, and the U-ring 24 of a seal ring 20 and a buffer ring 23 for forming a seal mechanism between the piston 11 and the cylinder tube 10 of the hydraulic cylinder of a hydraulic shovel, and a seal mechanism between a piston rod 12 and a rod extension part 12, are made as colored seal members, and the resin rings 20b and 23b, and the slidable ring 21 and bushing 22 of the seal ring 20 and the buffer ring 23 are made as colored slidable members. In addition, the colored seal members as well as the colored slidable members are differently colored for every hydraulic cylinder. According to this construction, an extent of damages to the seal mechanism of a specific hydraulic cylinder among a plurality thereof can be detected by confirming the color of a broken piece arrested with the filter of a return pipe.